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How bad is it to have 100% water as coolant during this time of year?

I’ve got a 97 Cavalier with a slow coolant leak. The engine started overheating a couple weeks ago on my way home from work, and when I checked the coolant reservoir it was empty, so my dad said it would be fine to refill it with water. I filled it with water to the top at the time and the reservoir has only lost a couple inches worth of water from the top since then. But now the weather report is showing overnight temperatures with windchill in the low 30’s over the next few days. My dad’s totally nonchalant about the 100% water, but I’m kind of panicking because I don’t want the engine block to freeze. Should I be worried?

Windchill is what it feels like when heat is removed from the skin. It has nothing to do with mechanical devices.

As long as the actual temperature doesn’t drop down to 32 degrees the water won’t freeze.


You only added a little bit of pure water into the cooling system. There is still plenty of anti-freeze in the coolant to handle the cooler temps. For the moment no worries.

However pure water is not a good coolant and pure water has no lubricants in it to protect the water pump bearings. So, you should get the leak fixed and then add the proper 50/50 mix of anti-freeze and distilled water.


I would have the cooling system pressure tested . . . might be something simple

BTW . . . how old is that coolant? Dexcool should be changed every 5 years

Are the electric fans coming on?

Are the thermostat and radiator cap original?

Fix the leak.
100% water is not good anytime of year.

Potential leaks, overheating, and straight water could all become a problem; an expensive one.
This needs to be repaired in spite of your dad’s nonchalant attitude about it as many major problems start out as a “so what” type of problem.

There is probably enough glycol in there to provide some protection from freezing, but if it’s really “pure water” that’s not good…Corrosion problems (as a result of the dissimilar metals in the cooling system and the minerals in the water) will get you if freezing doesn’t get you first.

Today’s motors are not designed to run exclusively with “pure” water ( distilled ?, as nothing else is really pure). Besides, It wan’t a big deal in bygone times when the life expectancy of motors was much less for a viariety of other reasons. So, using water alone was an ancillary factor to failing motors in non freezing conditions. Btw, the tap water and well water you use with all of it’s mineral content may cause it’s own problems. Well water especially that has not had particulates filtered out IMO, could be a recipe for early failure of coolant system seals and pumps. I would be interested to know what shops " really" used distilled water when changing coolant.
Water itself is an excellent “coolant” but today’s motors need more then just a coolant. Long life coolant contains too many necessary additives to be omitted or diluted for long.

You know that you cannot get through the winter without proper antifreeze in the system. You must find the problem and make repairs, the sooner the better. Topping up with water is a decent temporary measure while waiting for your appointment.

Do you have a dog? They love the sweet taste of ethylene glycol. It is a poison and even small amounts will kill the dog that licks it.

One teaspoon of antifreeze will kill a cat, 4 ounces a human being…fix the leak.

Fix the problem now, the overheating could kill your engine.

In summary of the other posts: The only concerns here are:

  1. Poisonous liquid on the street or garage floor.
  2. The lack of corrosion inhibitors in pure water. If your antifreeze is old, you had that problem anyway.

A little bit of antifreeze will prevent freezing until it gets really cold this winter. If the water looks colored, you still have a little antifreeze in it. If your windshield washer has pure water, that can be your indicator. A night that dips to 25F for a few hours will probably freeze the lines to the windshield washer nozzles, but not the reservoir. If the reservoir does not freeze, it is highly unlikely that the water in your radiator will freeze, and the water in the block won’t freeze until sometime after that.

As for other possible concerns - water has a higher specific heat than antifreeze/water mixture, so it is technically a better coolant, BUT water has a lower boiling point than coolant mix. Normally the boiling point is not a concern if the system holds pressure, (higher pressure = higher boiling point - that is how a pressure cooker works) but your system leaks, so pure water may boil under normal engine operation.

Antifreeze lowers the freezing point and raises the boiling point. The engine will freeze at 32 degrees and boil over at a lower temp due to the increased pressure within the closed system.